Amplifier arrangement for listening devices



Feb. 23, 1937. F. A. FISCHER AMPLIFIER ARRANGEMENT FOR LISTENING DEVICES Filed Sept. 26, 1935 INVENTQ'R #1. aka.

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. JA'JA o Patented Feb, 23, 193? A it ARRANGEMENT non LISTENING navrcss Fritz Alexander Fischer, Kiel, Germany, assignor to 'Electroacustic Gesellschaft mit beschrankter Hattung, Kiel, Germany, a firm Application September 26, 1935, Serial No. 42,261 in Germany (letober 29, 193d 3 Claims. (01. 177-386) My invention relates to an amplifier arrangement for listening devices for air and submarine sound reception, in which groups of electromechanical receivers for the sound are provided,

5 and by which the sound is converted into .electric energy, and in which further electric compensators are provided to compensate for the time lag which occurs through the different instances of time at which each of the individual receivers encounters the sound wave.

In such arrangements usually the receivers are connected in series with the compensator and in series with the latter is connected an amplifier which amplifies the combined energy of all receivers which is delivered by the compensator, and to the output of this amplifier is connected the sound indicator which has usually the form of a telephone. With such an arrangement of the amplifier the noises produced by the contact slides of the compensator are amplified to the same extent to which the received sound is amplified. This disadvantage might be avoided by placing the amplifier between the compensator and the sound receivers, instead of placing it between the compensator and the sound indicator. Such an arrangement is shown for instance in Figs. 4 and 6 of the United States patent to Fritz Lange No. 1,971,688. The disadvantage of this arrangement is, however, that a complete amplifier arrangement must be provided between each sound receiver and the compensator. Since usually for each receiver a number of thermionic tubes are used, and since usually in such a receiving arrangement quite a'number of sound receivers are 35 employed, the a number of amplifying devices would have to be increased to such an extent that 'theentire receiving arrangement is considerably complicated and rendered impractical.

It is the object of my invention to provide a 40 compromise between the two above-mentioned methods of amplifying by which it becomes possible to have the benefit of the advantages of the preliminary before it reaches the compensator without having the disadvantages of an amplification which occurs entirely between the compensator and the sound indicator. This compromise resides in dividing the amplification so that one or several steps of a preliminary amplifier are placed between the sound receivers and the compensator and the remainder of the amplifying devices is,

disposed betweenthe compensator and the sound indicator, such as a telephone. It is, of course, necessary for the preliminary amplification to amplification of the sound energy still use an individual amplifier tube or tubes for each sound receiver, while the amplifying device which is located between the compensator and the telephone may be common to all sound receivers. Any disadvantage which the individual 5 provision of a small amplifier for each sound receiver may have is compensated by a considerable lowering of the noise level in the telephone due to the compensator noises, and is further compensated by the advantage that the preliminary m amplifiers between the individual sound receivers and the compensator constitute non-reactive couplings if amplifier tubes with negatively biased grid are used.

My invention is illustrated in the accompany- 15 ing drawing in whicha Fig. 1 shows schematically the relative locations of the electro-mechanical impulse trans: lators or sound receivers, the preliminary amplie fiers, the compensator, the main amplifier and go the aural receiver; and

Fig. 2 shows in detail the wiring arrangement of a preliminary amplifier for one sound receiver.

Referring first to Fig. 1, a, b, c constitute the electro-mechanical impulse translators, shortly 25 called hereinafter electro-mechanical receivers which are suitably electrically connected in a manner shown for instance in the aforementioned Lange patent. Each of these receivers is connected to the compensator K by means of a so preliminary amplifier indicated in Fig. 1 by d, e, 2, respectively. The compensator is connected by way of the main amplifier g to the sound indicator h, which may for instance be a telephone. It will be noted that the main amplifier g amplifies the 35 entire energy delivered by the compensator k, and which constitutes the compensated combined energy of all receivers a, b, d.

In single chain compensators such as are shown in the Lange patent aforementioned all 40 sound receivers are connected to a single chain which chain is closed at one of its ends by means of a non-inductive resistance which is equal to the wave resistance of the chain, while the other chain end is connected to the amplifier, whose 45 input impedance must likewise be made equal to the wave resistance of the chain. Now-in order that none of the sound receivers may form a point of reflection for the energy of the other receivers, which-travels on the chain, each re- 50 ceiver must be balanced with respect to the chain so that its impedance, viewed from the compensator chain is large with respect to the wave resistance of the chain; Experiments have shown that it is suillcient to make the receiver imped- 55 inner resistance.

six times the wave resistance of the chain, If we consider this value, however, from the viewpoint of the receivers, it means that each receiver is closed'by the chain with one-sixth of its It-is, of course, entirely immaterial in such a case whether the receiver itself, for instance by suitably dimensioning its winding, is provided with such a resistance value, or whether the above required balancing ratio is accomplished by interposing a suitable transformer. With such an arrangement the receivers viewed from the standpoint of efliciency always operate with a considerable short-circuit. Only one-seventh of the electro-motive force of the receiver is supplied to the chain as the chain voltage. This loss in voltage through lack of proper balance may be avoided, however, by interposing between each electro-mechanical receiver and the compensator chain a coupling element which is free from reaction, and which connects the receiver through a resistance, which is large compared with the inner resistance of the receiver. An amplifier tube with negatively biased grid constitutes such a coupling element. Since such a tube constitutes a practically infinite resistance, the entire electro-motive force of the receiver may be brought into effect. If desired this electro-motive ,force may be considerably increased by the inter-position of a transformer. The ratio of the transformer is limited solely by the capacity of its secondary winding which eventually would short-circuit the highest frequencies of the frequency range to be transmitted.

Such an arrangement in which negatively biased amplifiers areused is shown in Fig. 2. In this figure the general arrangement, so far as the principle of preliminary amplification and main amplification is concerned is the same as that schematically shown in Fig. 1 In Fig. 2, a, b, c again represent the electro-mechanical impulse translators, or electro-mechanical receivers. Each of these receivers is provided with a preliminary amplifying device through which the energy delivered by it is transmitted to the compensator K. In this figure the details of this preliminary amplification are shown only with respect to the receiver 17. The receiver is connected by Way of a step-up transformer tr, to a thermionic amplifier tube e, the output side of which is connected to the compensator chain. The input side of this tube is connected to the secondary winding of transformer tr, a biasing voltage source i being provided in this circuit by which the grid of the tube is negatively biased. The output of the compensator which represents the'combined energy of all receivers, a, b, c is connected to the main amplifier g, the output side of which latter is connected to the telephone h.

In case of low-ohmic electro-mechanical receivers, for instance in case of electro-magnetlc or magneto-strictiVe receivers, it is not necessary to place these preliminary amplifiers into the immediate vicinity of the receivers, such as must be done in case of crystal receivers, which can be built only with a high-ohmic resistance. It is, therefore, of advantage to build these receivers with an inherently low-ohmic resistance and to connect these receivers directly into the lines which lead to the compensator and to locate the preliminary amplifiers in the vicinity of the compensator. It is still possible to construct for submarine sound receiving devices and at a frequency of from 500 to 3,000 employed in such 2,071,739 -ance when measured with respect to the chain,

arrangements and for a receiver resistance of the order of 100 ohms, transformers with a step-up ratio of about 1:75. On this possibility 'above all rests the great importance of the preliminary amplification.

It is unfortunately not possible to utilize to the fullest extent the amplification of the preliminary amplifier tubes, because with such an arrangement the tubes are here short-circuited to a certain degree in a manner similar in which in the arrangement without preliminary amplifiers the receivers operate with a partial short-circuit, as described hereinabove. Since the anode load resistance is equal to one-sixth of the inner tube resistance, the voltage amplification of the tube equals wherein D represents the effective anode reaction. It is, therefore, necessary to select an amplifier tube which has the smallest possible effective anode reaction. The limit in the selection of the anode reaction is determined by the requirement that the inner resistance which is inversely proportional to the effective anode reaction must not become too great, since it is not possible to make the no-load impedance of the transformer infinitely high. Therefore, tubes having too great an inner resistance are alone for this reason ill-fitted for this purpose.

I claim:-

1. In a sound listening device having a plurality of electro-mechanical receivers for receiv-' ing the sound energy and for translating it into electric energy, an electric compensator and a sound indicator, all connected in series, the method of amplifying the received sound energy, consisting of preliminarily individually amplifying the sound energy delivered by each receiver before it reaches the compensator and finally amplifying the combined energy of all receivers delivered by the compensator before it reaches the sound indicator, the total amount of amplification being divided between the preliminary and the final amplification in such proportion that the noise level in the indicator, due to the amplification of the compensator noises, remains below a disturbing magnitude.

2. In a sound listening device having a plurality of electro-mechanical receivers for receiving the sound energy and for translating it into electric energy, an electric compensator and a sound indicator, both connected in series, a preliminary thermionic tube amplifier for each sound receiver, having a negative grid bias, and being connected between the receiver andsaid compensator, and a main amplifier, connected between the compensator and the indicator, for finally amplifying the combined energy of all receivers before it reaches the sound indicator, the total amount of amplification being divided between the preliminary and the final amplification in such proportion that the noise level in the indicator, due to the amplification of the compensator noises, remains below a disturbing magnitude.

3. In a sound listening device having a plurality of electro-mechanical receivers for receiving the sound energy and for translating it into electric energy, an electric compensator and a sound indicator, both connected in series, a preliminary thermionic tube amplifier for each sound receiver, having a negative grid bias, and being connected between the receiver and said compensator, and a step-up transformer between each receiver and its preliminary amplifier, and a main amplifier, connected between the compensator and the indicator, for finally amplifying the combined energy of all receivers before it reaches the sound indicator, the total amount of amplification being divided between the preliminary and the final amplification in such proportion that the noise level in the indicator, due to the amplification of the compensator noises, remains below a disturbing magnitude.

FRITZ ALEXANDER FISCHER. 

